Method of separating the constituents of air



Oct. 2 0, 1936. s. TWOMEY 2,057,804

METHOD OF SEPARAI'ING THE CONSTITUENTS OF AIR Filed May 9, 1934 2Sheets-Sheet 1 FIG].

LEE 5. TWOMEY lNl/E/VTOR awe Q).

A TORNEY 6t 1935- L. s. TWOMEY 2,057,304

' METHOD OF SEPARATING THE CONSTITUEN TS OF AIR Filed May 9, 1934 2Sheets-Sheet 2 LEE 5. TWOMEY ATTY NE) 7pl/EN TOR Patented Oct. 20,- 1936PATENT OFFICE METHOD OF SEPARATING ENTS THE CONSTITU- OF AIR Lee S.Twomey, Vista, Calif.

Application May 9, 1934, Serial No. 724,690

24 Claims.

The object or my invention is to provide a I method for the separationof nitrogen, oxygen, and argon from the mixture of gases constitutingthe terrestrial atmosphere, in any desired state of purity, and also toprovide an apparatus suited to the performance of said method.

The attached drawings, Figs. 1 and 1a, are at once a flow sheet of themethod and a diagrammatic vertical section of a preferred apparatus. In.reading these drawings the right end of Fig. 1 should be joined to theleft end of Fig. 1a to form a single sheet, each figure showingsubstantially one half of the apparatus.

Fig. 2 is a diagram illustrating pipe connections by means of which themixed gas stream flowing from the dehydrating interchanger may be cooledby liquid refrigerant to control the temperature at the cold end of thedehydrating interchanger.

Elements of apparatus Referring to the drawings, the apparatus is seento consist of the following elements:

Element A is the primary air interchanger. .A shell H] adapted tooperate at a pressure somewhat above 60 pounds gauge is provided withtube sheets lll I, an air inlet pipe l2 communicating with any suitableair pump not shown, an air outlet pipe l3 communicating withinterchanger B, and a plurality of water drains l t-M having valvesl5-l5.

A multiplicity of tubes arranged between the tube sheets are dividedinto separate banks or groups I56, ll, !8, and it by means of upperheaders Z0, 2!, 22, and 23 and corresponding lower headers 24, 25, 26,and El. The order and arrangement of these banks is optional, but inpractice each bank consists of a large number of tubes of smalldiameter, closely spaced.

For producing high velocity of air flow over the tubes and angularimpingement of the air currents thereon I provide a plurality of thestaggered bailles indicated at 28-28.

While the drawings show but a single interchanger in element A, it isdesirable, if not necessary, to duplicate this element and to providebranched connecting lines with diversion valves to permit alternated'usefor the purpose of defrosting.

of which the general arrangement may be identical with that of theprimary interchanger. It consists of a shell 3| having tube sheets32--32 and three groups of tubes 33, 3t, and 35, these groups beingseparated by upper headers 36, 31,

Element B is the secondary air interchanger,-

and 38 and lower headers 39, 40, and ll. The shell is also provided withstaggered baflies 42-42.

Element C is the refrigerant evaporator, consisting of a shell 43adapted to a slight super-- atmospheric pressure, tube sheets 4444 and asufllcient number of tubes 45.

Element D is the nitrogen interchanger, consisting of a shell 46 adaptedto a slight superatmospheric pressure, a tube bank 41a with headersIll-41, and a plurality of baflles 4848.

Elements E and F are nitrogen Vaporizers iorming part of the transfercircuit, and consist of shells 69 and 50 adapted to about 55 poundsgauge pressure, upper headers 5| and 52, lower headers 53 and 54 andtube banks 55 and 56.

Elements G, H, and I are pure oxygen reboilers, consisting of shells 51,58 and 59 adapted to a low superatmospheric pressure, upper tube headera60, El and 52, lower tube headers 63, 64 and 65, and tube banks 66, Siand 68.

Element J is the low pressure fractionating column, having a shell 69adapted to a low superatmospheric pressure. This shell is continuouswith and supported by the shell 16 of high pressure column K. The twoshells are nonleakably separated by a condensing unit consisting of tubesheets 'H H, a head 12 and a plurality of tubes 13. The condenser thusformed is partially submerged in liquid accumulating in the lower end ofthe upper shell, while the interior of the tubes drains into the upperend of the lower shell.

A substantial part of the height of shell 69 is filled with spacedevaporating plates, which are shown in the drawings in two of thenumerous optional types. At M I indicate the conventional disc anddoughnut plates, while indicates the well known bubbling plate which isprovided with vapor nozzles 16 and bubble caps ill. The use of suchplates for condensation and fractional reevaporation of mixed vapors iswell known and understood and any type of plate or grid known in the artmay be used, though in practice I prefer a bubbling plate.

Element K is the high pressure fractionating column, consisting of ashell 10 provided with disc and doughnut plates 18 or bubbling plates 19as above described. It is also provided near its upper end with anannular pocket termed the nitrogen receiver.

Element L is the argon rectifying unit, consisting of a shell 8| whichis closed at its upper end 82 and open at its lower end into theinterior 65 pipe 98.

of column J. This shell is provided with any preferred form of bubblingplates 88.

Element M is the transfer unit comprising a transfer interchanger, awater cooled coil and a gas pump or compressor. The interchangerconsists of a shell 85, upper and lower tube headers 8686 and a tubebank 81. The gas compressor 88 should be capable of operating at about55 pounds discharge head. The lower tube header 86 communicates with thesuction side of this compressor, which discharges through cooling coil89 into the lower end of shell 85. It is desirable to provide this shellwith staggered baffles 99-99. The coil is immersed in a water bath 9|whichis supplied with cold water through a pipe 92 and overflows heatedwater through Functioning of apparatus The above apparatus, whenoperated in the manner hereinafter described, functions as follows, itbeing understood that all temperatures are stated in degrees Kelvin(Centigrade degrees absolute) and all pressures in pounds aboveatmosphere.

Air which has previously been freed from carbon dioxide and dust isintroduced into interchanger A through pipe I2, at atmospherictemperature and at such pressure as will cause the flow of air at thedesired rate to and into high I pressure column K. The pressure withinthis column being somewhat under 60 pounds gauge, the pressure at whichair is delivered into interchanger A will be column pressure pluspressure drop through intervening apparatus, which will vary withdetails of construction.

Coming into contact with the various tube banks in this interchanger,which are cooled by the final product gases in a manner to be described,the air is precooled to 200. The water is thus substantially removedfrom the entering air, a portion of this water being drained throughpipes I4 as it accumulates. A portion of the water collects as frost inthe portion of the shell which is below the freezing point, from whichit is periodically removed by alternating and thawing.

Passing to interchanger B through pipe I3 the air flows over tube banks33, 34, and 35 and is reduced in temperature to 96. It now passes to thebottom of column K through pipe 94, entering the column in gaseous format 96 and column pressure. This pressure will be the vapor pressure ofthe nitrogen condensing in the upper portion of the column plus liquidhead on the plates, or more or less 58 pounds gauge at the point ofentry of the air flow.

The course of the air feed flow through the apparatus is indicated bythe directional arrows I.

A stream of liquid nitrogen at 116 and at a pressure approximating 300pounds, drawn from a source of supply not shown, is introduced throughpipe 95 into nitrogen receiver 80. The quantity admitted is regulated byvalve 98 and the evaporationconsequent to reduction of pressure to thatof the column reduces the temperature of the nitrogen entering thereceiver to 94. The condenser tubes 53 are maintained at a temperatureof 92 by the crude oxygen bath, as will be described, and at thistemperature and the pressure existing within the tubes, nitrogen iscondensed and drains into the nitrogen receiver. A portion of the mixedcondensate and nitrogen feed constantly overflows to reflux thefractionating plates below.

The supply of liquid nitrogen may be produced in any desired manner, butI find it highly economical and advantageous to utilize the apparatusand method described in my copending application, Serial No. 724,691,filed May 9, 1934 and entitled Method of producing low temperaturerefrigeration.

The temperature at the base of column K being 96 while the top of thecolumn has a temperature determined by the boiling point ofsubstantially pure nitrogen at flve atmospheres absolute, or about 94,the fractionating plates function in the well known manner to condense acrude oxygen (concentration 35% to 40%) which collects in a pool 91 inthe bottom of the column.

While it is desirable to introduce the nitrogen supply into receiver asabove described, it is also permissible to introduce it into nitrogenoutlet pipe 98, as through pipe 95a and valve 96a. The advantage inintroducing it into the tower as shown is that the vapor produced byreduction of pressure from 300 pounds to that of the column isrecondensed and a quiet flow through pipe 98 is produced.

All of the nitrogen of the entering air save that retained in the crudeoxygen, plus that introduced through pipe 95 as above, is ultimatelycollected as a substantially oxygen-free liquid in receiver 88, fromwhich it passes through pipe 98 to and through the tubes of reboiler Iwhere it is cooled against liquid oxygen to 92. From this unit is passesthrough pipe 99 to the tubes f interchanger D where it is cooled againstgaseous final product nitrogen to 80, thence through pipe I to the tubesof vaporizer E where it is cooled against vaporizing nitrogen in thetransfer circult to 79, thence through pipe Iill to a point in lowpressure column J above the stack of fractionating plates. Valve I92 inpipe Illl maintains the pressure in the above system at that of the highpressure column, about 58 pounds. On passing this valve the pressure isreduced to that of the low pressure column, about 3 pounds. At thispressure and a temperature of 79 the nitrogen feed is retainedsubstantially in liquid form until it reaches the fractionating platesin this column.

The above described flow of nitrogen through the apparatus is indicatedin the drawings by directional arrows 2.

The crude oxygen collecting in pool 91 passes through pipe I03 to thetubes of reboiler H where it is cooled against liquid oxygen to 93,thence through pipe I94 to the tubes of vaporizer F where it is cooledagainst vaporizing nitrogen in the transfer circuit to 81, thencethrough pipe I95 to a medial point in low pressure column J. A pressureequivalent to that of the high pressure column is maintained in thissystem by valve I96 and on passing this valve the pressure is reduced tothat of the low pressure column. At the temperature and pressure ofintroduction, 81 and 3 pounds, the crude oxygen enters the column as aliquid.

Attention is here directed to the importance of maintaining both thenitrogen and oxygen feeds to the column substantially entirely in liquidphase, for which due provision is made in the apparatus hereindescribed.

It should be noted that to obtain the best results the crude oxygen feedto the low pressure column should be introduced at a point in its heightwhere the constitution of the feed is substantially identical with thatof the liquid on the entering plate.

by the directional arrows 3.

The temperature at the base of column J being" maintained at 91 /92, theboiling point of pure oxygen at 0.2 atmosphere pressure, and that of thetop of the column being held at 78 by the reflux nitrogen admittedthrough valve I02, a close fractionation is effected on the plates and apool of pure oxygen I01 collects at the base of the column. This pool isin liquid communication through pipes I08, I09, and III! with similarpools maintained around the tubes in reboilers I, H, and G, the slightlyhigher temperatures of the tube banks in these reboilers supplementingthe reboiling effect of condenser tubes I3, the evolved gas returning tothe column through pipes III, H2, and H3 in series.

' Pure oxygen is withdrawn from the column through pipe 6, passingthence to header 40 and tube bank 3d in interchanger B where it absorbsheat from the precooled air, thence through pipe I I5 to header 22 andtube bank I8 of interchanger A where it is brought to a temperature say3 C. below atmospheric in precooling the warm air feed, being finallydelivered through pipe H6 at substantially atmospheric pressure to apure oxygen storage vessel, not shown.

The flow of pure oxygen through the apparatus is indicated by thedirectional arrows s.

Pure nitrogen in gaseous form leaves column J at 79 through pipe Illandpasses into the shell of interchanger D, in which it is heated to inwithdrawing heat from the liquid nitrogen fiowing from the high pressureto the low pressure column, thence through pipe H8 to header 2| and tubebank 35 in interchanger B where it is heated to 200' by interchangeagainst precooled air, thence through pipe H9 to the tubes 45 ofrefrigerant evaporator C, where it is cooled to more or less by theevaporation of refrigerant around the tubes, and finally through pipeI26 to header 23 and tube bank IQ of interchanger A, where it is heatedto say 3 below atmosphere by interchange against the warm air feed. Thepure nitrogen is finally delivered at substantially atmospheric pressurethrough pipe I2I to a pure nitrogen storage vessel, not shown. The abovedescribed fiow follows directional arrows 5.

The transfer circuit is a closed cycle which may be charged with anysuitable gas, assumed in the present instance to be nitrogen. Compressor83 takes the gas at more or less atmospheric pressure and. temperatureand delivers it at say 55 pounds pressure into the watercooled coil 89,where the heat of compression is removed. From the coil the gas passesthrough pipe I22 into the shell 85 of the transfer interchanger where itis cooled against returning cold gas and issues from the shell at 96.Passing now through pipe I23 it enters the tubes of reboile'r G where itis cooled and condensed by the pure oxygen bath to 92. From these tubesit passes through pipe I26 to the U- pipe I25 where the flow isdivided'by valves I26 and I2? between the shells t9 and 5b of VaporizersE and F. Valve I26 reduces the pressure in the shell of vaporizer E tosubstantially atmospheric and the temperature to 77, at whichtemperature a portion of the nitrogen is liquid and forms a bath aroundthe tubes. Valve I21 delivers the liquid into the shell of vaporizer Fagainst a back pressure of 6 pounds, which is maintained by regulationof outlet valve I28, the temperature of the liquid bath so formed being81.

The gas vented from these shells is collected by U-pipe I29 and returnedthrough pipe I33 to the tube bank 81 of the transfer interchanger.

The returned gas enters this tube bank at a tern perature between 77 and81 (depending on the relative regulation of valves I26, I21, and I28),absorbs heat from the warm gas delivered from the compressor through thewater cooled coil,

and passes through pipe I30 to the suction of the compressor atapproximately atmospheric temperature.

This completes the closed transfer cycle, the flows through which areindicated on the drawings by directional arrows 6.

The refrigerant evaporator C is suppliedwith a suitable liquefied gas,as for example ethylene, through pipe I3I, at a temperature (in the caseof ethylene) of 170, from any source of supply not shown. The quantityadmitted is so controlled by expansion valve I32 that evaporation of theliquid refrigerant will reduce the temperature of the pure nitrogenpassing through the tubes 65 of this evaporator to more or less Thegaseous refrigerant then passes through pipe I33 to tube bank l6 ofinterchanger A, where it assists in precooling the warm air supply, andis finally delivered through pipe I34 to be returned to the liquefyingmeans not shown. The flow through the refrigerant circuit, in so far asthis cycle is shown in the drawings, is indicated by directional arrows7.

While the use of ethylene in this circuit is preferable, it ispermissible and may in some cases be desirable to utilize otherliquefied gases, as for example liquid methane or nitrogen drawn fromthe source which supplies the high pressure column.

In the modification illustrated in Fig. 2 the stream of air leavingdehydrating interchanger A is cooled by the evaporation of a liquidrefrigerant to control the temperature of the air column J that its openlower end is at the level where the greatest concentration of argonoccurs in the vapor. The upper end 82 of shell 8i being surrounded by anatmosphere of lower temperature than the condensation point of oxygen,the bubbling plates within the argon column are constantly refluxed witha condensate which, as it fiows downwardly from plate to plate, becomesconstantly richer in oxygen. this condensate finally being returned tothe plates of column J.

The gas escaping from the top of column L consists of argon in acommercially useful admixture with other atmospheric gases. This mixturepasses through pipe I35 to header 39 and tube bank 33 of interchanger B,where its heat absorbing capacity is utilized in further cooling theprecooled air supply. From this tube bank it passes through pipe I36 toheader 20 and tube bank ll of interchanger A, where it assists inprecooling the warm air supply, finally issuing from pipe I37 atsubstantially atmospheric temperature as the argon product. The courseof this flow is indicated by the directional arrows 3.

Elements of control As substantially all of the refrigeration requiredto offset. leakage of heat into the apparatus is produced by theintroduction of liquefied and precooled nitrogen into the high pressurecolumn, the capacity of the apparatus is limited solely by thefractionating capacity of the two columns and by the area ofinterchanging surface, both being fixed in the original design. Belowthis predetermined limit the throughput of the apparatus may be variedat will by balancing the discharge of the air supply pump against thequantity of liquid nitrogen admitted through valve 96 into the highpressure tower.

The additional refrigerating effects produced by the passage of arefrigerant through evaporator C and by the expansion of nitrogen in thetransfer circuit are negligible in point of quantity and are designedsolely to afford controls through which optimum operating conditions maybe maintained.

It will be obvious that to maintain constant conditions throughout theapparatus, not only the volume of air entering the high pressure columnbut also its temperature must be held level, and that the attempt tocompensate variations in the temperature of the warm air feed byregulation of the liquid nitrogen supply would require simultaneousreadjustment of all the controls.

The introduction into the system of the refrigerant evaporator providesa ready and simple means for compensating variations in temperamm ormoisture of the warm air feed, a single manual or temperature-responsivevalve controlling the amount of refrigerant evaporated in heat exchangerelation with the stream of pure nitrogen prior to'its entry to theprimary air interchanger and thus directly controlling the heatabsorbing capacity of this interchanger. This completely independentcontrol permits the regulation of the temperature of the air stream inthe precooling'stage and without disturbance to the remainingadjustments.

This regulation is preferably applied to the pure nitrogen streambecause of its materially greater volume and heat carrying capacity ascompared to other of the final products, but by providing for a greatertemperature range in the control it could be as well applied to thefinal product oxygen, or less desirably it might be applied to the airsupply itself.

A material advantage attendant on the introduction of a minor andclosely controllable'refrigerating effect into the system at the upperend of the primary interchanger is the retention of all materialformation of ice within this interchanger, which is designed for theremoval of ice whenever it accumulates to such point as to beobjectionable. If the temperature at the upper or air-outlet end of theprimary interchanger is allowed to fluctuate materially, the zone ofmaterial ice formation is at times liable to advance into the secondaryinterchanger, from which frost cannot be removed without shutting downthe entire apparatus.

The purpose of a controlled refrigerating effect, in whatever mannerapplied, not only compensates changes in the temperature of .theentering air due to seasonal changes in the temperature of cooling waterused to remove the heat of compression, but also provides controllableheat absorbing capacity for condensing and freezing a variable moisturecontent which is also seasonal.

The transfer circuit has three distinct functions. First, it adds amaterial amount of heat to the pure oxygen pool in reboiler G and thusaugments the reboiling action of condenser tubes 13. This reboilingeffect may be quantitatively varied by controlling the combined openingsof valves I26, I21 and I28, thus regulating the amount of the warmer gaspassing through the coil of reboiler G.

Second, it affords a means for controlling the heat gradient within thelow pressure column by varying the relative temperatures of the liquidnitrogen and the crude oxygen passing toward expansion valves I02 andI06 respectively. This temperature variation is produced by diverting aportion of the transfer fluid from one to the other of vaporizers E andF, by which diversion the cooling effect applied to one of the streamsis increased at the expense of the other.

The third and perhaps the most important function of the transfercircuit is to effect a supercooling of the nitrogen reflux, in itspassage through interchanger E, by which it is maintained in a whollyliquid condition after it passes through valve I02 into the lowerpressure zone within column J. By preventing ebullition of the reflux atthe point of pressure release, spattering and entrainment of the liquidinthe outflowing pure nitrogen stream are avoided, the operation of thecolumn is steadied and its capacity materially increased, and the purityof the final product nitrogen is greatly enhanced. The same effect onthe crude oxygen feed is produced in the same-manner in interchanger F.

Reservations It should be noted that both the apparatus and the methodsabove described may be varied inseveral particulars without departingfrom the spirit of the invention.

The interchangers, for example, are shown in a vertical position andreference is made to the top and the bottom of these elements indescribing the various flows through them. It will be understood thatthe descriptions are so framed for convenience only, and that with dueregard to feasible construction these elements may be placed eithervertically or horizontally and that, with the possible exception of theprimary interchanger A, the flow directions may be reversed so long ascounterflow is maintained. This reservation, however, will not apply toreboilers or vaporizers in which a pool of liquid is maintained.

In the drawings the oxygen reboilers are shown connected serially. Theseelements have no interdependent function and each may be independentlyattached to the column by a liquid pipe and a vapor pipe, atsubstantially the level indlcated.

The argon column L may be omitted if it is not desired to remove theargon, without any fur ther change in the apparatus than the omission ofthe corresponding headers and tube banks from interchangers A and B.

The temperatures and pressures disclosed are intended to beillustrative, though they are an accurate disclosure of desirableconditions for the simultaneous production of pure nitrogen and pureoxygen in the apparatus shown. If either or both of the main productsshould be desired in a less pure state, the apparatus is fully capableof regulation to that end, usually with some increase in its throughputcapacity, with a corresponding variation from thedescribed temperaturesand pressures.

The method of fractionating shown may be used for the separation of anymixture of liquefled gases in which the boiling point of the higherboiling constituent at the pressure carried on the low pressure column(which may be either above or below atmospheric) lies below the criticaltemperature of the lower boiling. An example is' the fractionation of anargon-nitrogen mixture, in which a substantially complete separation maybe made in the described apparatus but by the use of other temperatures.

Advantages claimed The ease and completeness with which the apparatusmay be regulated and the most desirable operating conditions maintainedhas already been pointed out.

By reason of the application of the main refrigerating effect in theform of a liquefied and supercooled gas (nitrogen) of very low boilingpoint, it is possible to utilize a highly economical method of producingthis refrigeration, such as that described in my copending applicationSerial No. 724,691, thus greatly reducing the power consumption per unitof product. An actual apparatus on a large working scale is rectifyingair to substantially pure oxygen and nitrogen with a power consumptionof 175 H. P. per 1,000 cubic feet of free air per minute, whereas thebest previous practice of which I am aware requires 289 H. P. forthesame amount of air and produces a decidedly less closefractionation.

By reason of the introduction of the main reirigerant supply directlyinto the high pressure column, the large amount of interchange surfacerequired for the initial condensation of the air is avoided and theapparatus is greatly reduced in first cost. For the same reason, thetime required for starting the apparatus from room temperature is muchreduced as, assuming a suificient supply of the refrigerant to beavailable, the reduction in cooling down time is practically onlylimited by the ability of the apparatus to withstand sudden coolingwithout damage.

The most important advantage of the apparatus and method over'thedisclosures of the prior art lies in the simultaneous production of thetwo main products, oxygen and nitrogen, in a state of purity. It hasheretofore been possible to obtain pure nitrogen and impure oxygen, orvice versa, but to the best of my knowledge no method or apparatus shownin the prior art has simultaneously yielded both of these products in acommercially pure state. The nitrogen yield of the above apparatus, inactual operation under the described conditions, has a purity of 99.8%to 100% whentested by the most approved methods, while the oxygen yieldshows a purity of 99.8%, the impurity being a trace of argon. The argonyield is in the neighborhood of 70% of the quantity existing in the airsupplied, and the purity is within commercial requirements, theprincipal contaminating body being oxygen.

The term binary mixture used in some of the attached claims is intendedto include mixtures containing two major constituents which it isdesired to separate, together with relatively small quantities ofvarious other gases which in the fractionation steps pass into one orthe other of the major constituents. Thus air is a binary mixture in thespirit of the claims which call only for the separation of oxygen fromnitrogen.

I claim as my invention:

1. In' anair fractionating process including the separation ofsubstantially pure nitrogen from crude oxygen in a first fractionatingzone maintained at relatively high pressure and the refractionation ofsaid products in a second fractionating zone maintained at a relativelylow pressure, the steps comprising: maintaining a third fractionatingzone in direct heat exchange relationship with-said second zone, saidthird zone communicating with said second zone at only the lower end ofsaid third zone; withdrawing from a medial position in said second zonea stream of mixed gases including oxygen and argon; cooling said thirdzone by direct heat transfer to said secand zone to produce a condensatecontaining argon and oxygen; subjecting said condensate to repeatedfractionation whereby gaseous argon is separated from said condensate;withdrawing. gaseous argon from the upper part of said third zone, andreturning to said second zone a condensate containing less argon thanthe gas withdrawn into said third zone.

2. The method of operating a multiplate frac tionating column for theseparation of the lowerboiling constituent from a mixture of gases,which comprises: introducing said mixture, in substantially whollygaseous form and at a temperature approximating its point of incipientliquefaction, into said column at a point below the plates therein;maintaining the upper end of said column at the condensation temperatureof the pure lower-boiling constituent and thereby producing acondensate; refluxing a portion of said con densate over said plates toeffect condensation of said mixture; subjecting the mixed condensates torepeated fractionations on said plates whereby a condensate rich in thehigher-boiling constituent is collected in the lower end of said columnand the pure lower-boiling constituent passes in gaseous form into theupper end of said column; withdrawing last said condensate in liquidform from the lower end of said column, and condensing, collecting andwithdrawing pure lower-boiling constituent from the upper part of saidcolumn.

3. In the operation of a two stage air fractionating column in which thestages operate at different pressures, the step of introducing the airfeed in substantially wholly gaseous form below the plates in the higherpressure stage in said column.

4. The method of controlling cold-end temperature in the dehydrationstep of a mixed-gas separating operation which comprises: passing astream of said mixed gas successively through a zone in which thetemperature of said gas is reduced to a point below the freezing pointof water and through a zone in which the temperature of said stream isstill further reduced; returning a stream of a separated gas from saidseparating operation into heat exchange relation with said mixed gasstream successively in last said zone and in first said zone, andevaporating a controlled supply of a liquid refrigerant in heat exchange relation with one of said streams at a point intermediate saidzones.

5. A method substantially as and for the purpose set forth in claim 4,in whichthe mixed gas is air and the refrigerant is liquefied ethylene.

6. A method substantially as and for the purpose set forth in claim 4,in which the refrigerant is applied to the mixed gas stream.

7. A method substantially as and for the purpose set forth in claim 4,in which the refrigerant is applied to the returning gas stream.

8. In an air fractionating operation involving the collection of anoxygen-rich condensate in a streams of condensate to difierenttemperatures and to such temperatures as to maintain each said streamsubstantially wholly liquid as it passes into said low pressure zone.

10. A step substantially as and for the purpose described in claim 9, inwhich'the refrigeration required for supercocling said stream issupplied by a liquid refrigerant introduced into said fractionating.operation from a source extraneous to said fractionating operation.

11. In a mixed-gas fractionating operation involving the feeding of astream of condensate maintained at relatively high pressure into afractionating zone maintained at relatively low pressure with attendantrelease of the pressure on said stream, the steps comprising:supercooiing said stream of condensate by heat exchange against boilingliquid nitrogen, and so controlling the pressure on said boilingnitrogen as to reduce said stream to a temperature at which its pressuremay be reduced to that of said low pressure zone without materialebullition of said stream at its point of entry into said low pressurezone.

multiple fractionating column, the steps comprising: introducing saidair in gaseous form into the lower end of said column and introducing astream of liquid nitrogen into the upper end of said column to act asrefiux therein, the nitrogen so introduced being additional to anynitrogen produced by the fractionation of said air and being at leastsufficient in quantity to compensate infiltration of heat into saidcolumn.

13."Ihe method of separating the constituents of a binary mixture ofgases, which comprises: introducing said mixture in precooled gaseousform into a first fractionating zone maintained at a relatively highpressure; separately condensing and collecting in said first zone acrude condensate rich in the higher-boiling constituent of said mixtureand a pure condensateconsisting of the lower-boiling constituent insubstantially pure form; introducing said crude condensate into a secondfractionating zone maintained at a relatively low pressure; transferringheat from said first zone to said second zone to produce condensation insaid first zone; introducing said pure condensate into the upper part ofsaid second zone to act as reflux therein and simultaneously introducingfrom an extraneous source into the upper part of said second zone afurther quantity of a liquid substantially identical with said purecondensate, said further quantity being at least sufiicient tocompensate infiltration of heat into both said zones.

14. A method substantially as and for the purpose set forth in claim 13,in which the further quantity of liquid is intermixed with said pure 12.In the continuous fractionation of air in a condensate inlsaid firstzone and travels with said condensate into 'said second zone.

15. A method substantially as and for the purpose set forth in claim 13,in which the further quantity of liquid is intermixed with said purecondensate at a point between said first zone and said second zone andtravels with said condensate into said second zone.

16. A method substantially as and for the purpose set forth in claim 13,in which the further quantity of liquid andsaid pure condensate areseparately and simultaneously introduced into the upper part of saidsecond zone.

17. In a method of fractionating air involving the production of anoxygen-rich condensate and liquid nitrogen in a first fractionating zonemaintained at a relatively high pressure and the production ofsubstantially pure gaseous nitrogen and gaseous oxygen in a secondfractionating zone maintained at a relatively low pressure; a closedheat transfer cycle comprising: compressing a stream of gaseous nitrogenand removing the heat of compression; cooling said gaseous nitrogen byheat exchange against returning gaseous nitrogen within said cycle;liquefying said gaseous nitrogen by heat exchange against pure liquidoxygen condensing in said second zone, whereby said pure oxygen isreboiled; evaporating said liquid nitrogen under a pressure less thanthe compression pressure by heat exchange against a liquid moving fromsaid first zone to said second zone, whereby said liquid is cooled, andreturning the evaporated nitrogen to be again compressed.

18. In a method of fractionating a mixture of gases involving theproduction of condensates of different boiling points in a firstfractionating zone maintained at relatively high pressure, the separatetransfer of said condensates to a second fractionating zone maintainedat relatively low pressure and the production in said second zone of acondensate consisting of only the higherboiiing component of saidmixture, the heattransfer steps comprising: passing a compressed andcooled gaseous refrigerant in heat exchange relation with last saidcondensate, whereby said gaseous refrigerant is liquefied and last saidcondensate is reboiled; reducing the pressure on said liquefiedrefrigerant and passing said liquefied refrigerant in heat exchangerelation to first said condensates, whereby first said condensates arecooled and said liquefied refrigerant is returned to the gaseous state.

19. In the continuous fractionation of air in a multiplate fractionatingcolumn, the step of applying a refrigerating effect to said column bythe evaporation of liquid nitrogen additional to any nitrogen producedby the fractionation of said air. a

20. In the continuous fractionation of a mixed gas in a multiplatefractionating column, the step of applying a refrigerating effect tosaid column by the evaporation of a liquid refrigerant of substantiallythe same composition as the lowest boiling constituent of said mixture,said refrigerant being additional to any quantity of said constituentproduced by said fractionation.

21. In a method of fractionating a mixture of gases involving theproduction of condensates of different boiling points in a firstfractionating zone maintained at relatively high pressure, the separatetransfer of said condensates to a second fractionating zone maintainedat relatively low pressure and the production in said second zone of acondensate consisting of only the higherboiling component of saidmixture, the heattransfer steps comprising: passing a compressed andcooled gaseous refrigerant in heat exchange relation with last saidcondensate, whereby said gaseous refrigerant is liquefied and last saidcondensate is'reboiled; reducing the pressure on said liquefiedrefrigerant and passing said liquefied refrigerant in heat exchangerelation to one of first said condensates; whereby last said condensateis cooled and said liquefied refrigerant is returned to the gaseousstate.

22. In a mixed-gas fractionating operation involving the feeding of astream of condensate maintained at relatively high pressure into afractionating zone maintained at relatively low pressure with attendantrelease of the pressure on said stream, the steps comprising:supercooling said stream of condensate by heat exchange against aboiling liquid refrigerant, and so controlling the pressure on saidboiling refrigerant as to reduce said stream to a temperature at whichits pressure may be reduced to that of said low pressure zone withoutmaterial ebulliti'on of said stream at its point of entry into said lowpressure zone.

23. Ina mixed-gas fractionating operation involving the feeding of astream of condensate maintained at relatively highpressure into afractionating zone maintained at relatively low pressure with attendantrelease of the pressure on said stream, the step of supercooling saidstream of condensate to a temperature at which its pressure may bereduced to that of said low pressure zone without material ebullition ofsaid stream at its point of entry into said low pressure zone.

24. The method of controlling cold-end temperature in the dehydrationstep of a mixed-gas separating operation which comprises: passing astream of said mixed gas successively through a zone in which thetemperature of said'gas is reduced to a point below the freezing pointof water and through a zone in which the temperature of said stream isstill further reduced; returning a stream of a separated gas from saidseparating operation into heat interchange relation with said mixed-gasstream successively in last said zone and in first said zone, andapplying a cooling step to one of said streams at a point intermediatesaid zones.

LEE S. "I'WOMEY.

